In general, physical methods, especially cathode sputtering, have been used to date for the production of thin copper layers. However, this method has the disadvantage that, for example in the production of copper starting layers for the interconnect system of highly integrated circuits—with increasing reduction of the geometrical dimensions—uniform closed surface layers are no longer obtained. Regarding the coating of patterned substrates, for example seed layers for ULSI interconnects, only poor conformality is achieved with physical methods like PVD and voids can be created, which may, for example increase the line resistance. In the case of spintronic layers stacks, ferromagnetic films as well as non-magnetic spacer layers produced by sputtering processes and possibly by the molecular beam epitaxy (MBE) technique, the GMR effect decreases dramatically due to the softening of interfaces by unwanted sputtering or intermixing effects.
Chemical methods, for example, variants of chemical vapor deposition (CVD), D are suitable as alternative methods for producing such layers. Here, source substances or precursors which contain the desired metal (e.g. copper) in the form of a chemical compound are fed in the gaseous state to a vacuum chamber which is in the form of a hot- or cold-wall reactor and in which the layer deposition is subsequently effected. For this purpose, the precursors are converted into the gas phase prior to deposition. Accordingly, a layer formation reaction takes place on the surface of the heated wafer substrate. This may consist in targeted thermally controlled decomposition of the precursor; often, reducing or oxidizing agents are also necessary for the layer deposition. However, the CVD methods have the disadvantage that the layer growth is not uniform here and closed surface layers form only from a thickness of a few 10 nm.
By using atomic layer deposition (ALD), these disadvantages can be avoided. ALD is a cyclic method in which two reactants may be fed to the reaction chamber in pulses. The corresponding pulses are separated from one another by inert purging and/or evacuation steps so that the two reactants never meet one another in the gas phase and exclusively surface reactions of the second reactant with adsorbates of the first reactant lead to layer formation. The first reactant is initially chemisorbed on the substrate surface so that the substrate is substantially covered with a monolayer of the precursor. Further monolayers which form by physisorption are removed during the purging or evacuation pulses. It is therefore necessary for the precursor to be able to undergo chemisorption on the substrate to be coated. After introduction of the second reactant, the desired films are obtained with layer thicknesses typically less than one monolayer (per cycle). By means of the ALD method, it is therefore possible to control the desired layer thickness very accurately via the number of ALD cycles; highly conformal layers are obtained.
In order to produce copper layers by means of ALD, in general two approaches can be chosen for the deposition:
Either elemental copper can be produced directly during the ALD process with reducing agents. However, for reducing the Cu in the precursor molecules, high process temperatures >300° C. with molecular hydrogen H2 as co-reactant are necessary. Thus, the formation of ultrathin (<10 nm) and continuous copper films is almost unrealizable due to the strong tendency of copper for minimizing the surface and interface energy by formation of spherical copper particles, particularly on oxides (e.g. SiO2, a typical substrate in TSVs) or transition metal nitrides (e.g. TaN, a typical copper diffusion barrier in ULSI devices), which is reasoned by huge surface energy mismatches. The agglomeration tendency can be avoided by using lower process temperatures and stronger reducing agents, like atomic hydrogen generated by plasma discharges. Nevertheless, plasma processes are incapable to achieve conformal reductions in high aspect ratio structures due to the preferred reaction of the atomic hydrogen on free areas and less on shadowed areas, deep trenches or vias.
On the other hand, elemental copper films can be realized by the reduction of intermediate ALD copper compounds. The reduction can be a part of the ALD process itself or fulfilled within a subsequent reduction process. Intermediate copper compounds may be copper nitride or copper oxide. Usually, these ALD processes can be handled at lower temperatures for avoiding the agglomeration of the ultrathin ALD films and do not require plasma-based processing.
U.S. Pat. No. 6,869,876 B2 describes a method in which first a copper halide layer is produced on the substrate with copper(I) and copper(II) complexes as a precursor; this layer is then reduced by means of a reducing agent, for example diethylsilane, to give a copper layer.
U.S. Pat. No. 6,482,740 B2 describes an ALD method in which a copper oxide layer is first obtained. Here, copper(I) and copper(II) compounds, for example, (PEt3)Cu(hfac) are used as a precursor. For producing the oxide layer, in each case an oxidation pulse is carried out during an ALD cycle. For reducing the copper oxide layer, a reduction at temperatures higher than 300° C. is effected.
The above methods have the disadvantage that fluorine-containing precursors are used. Fluorine can accumulate at the interface with the substrate material and reduce the adhesion of the copper layer to the substrate there.
US 2010/0301478 A1 discloses an ALD method in which 18 valence electron copper β-diketonate precursors and similar precursors are used which are not fluorine-containing. Intermediate copper oxide layers are reduced by means of a reducing agent.
According to one aspect, there is still a need to find a highly efficient thermal reduction process for reducing ultrathin copper oxide films to metallic copper on arbitrary substrates. According to a further aspect, there is a need for low temperature processes for avoiding film agglomeration.